1. Field of the Invention
The present invention relates to an optical pickup having the function of recording information signals on an optical information record medium (hereafter, simply referred to as “optical disk”) or reproducing information which has been recorded on an optical disk by irradiating a spot beam on a recording surface of the optical disk, and an optical information recording/reproducing apparatus equipped with the optical pickup.
2. Prior Art
Optical pickups are generally configured to detect a focus error signal and a tracking error signal, and control the position of an object lens with use of the error signals so that a converged beam spot (hereafter, also referred to as a “convergence spot”) can be placed correctly on a proper recording track of an optical disk. As typical methods for detecting the tracking error signal, a 3-spot method, a push-pull method, a differential push-pull method (hereafter, referred to as a “conventional DPP method” for the simplicity of explanation), etc. are well known.
Especially, the conventional DPP method, having the advantage of precisely detecting the tracking error signal with a relatively simple optical system while realizing the detection of a reliable tracking error signal from which offsets (caused by displacement of the object lens and a tilt of the optical disk) have been removed satisfactorily, are widely employed mainly for optical pickups for recordable/rewritable optical disks in recent years (see JP-A-7-272303, for example).
In the following, the principle adopted by the conventional DPP method for the detection of the tracking error signal will be explained briefly with reference to FIG. 1. Incidentally, FIG. 1 will also be used later for the explanation of the present invention because of illustrating a construction element of the invention. As shown in FIG. 1, an optical pickup employing the conventional DPP method incorporates a diffraction grating 2 which is placed between a semiconductor laser light source 1 and a half mirror 3. The diffraction grating 2, which is generally provided with a plurality of linear grating grooves at even pitch as shown in FIG. 2, has the function of diffracting and separating a laser beam emitted from the semiconductor laser light source 1 into at least three beams containing a 0th order beam and ±1st order diffracted beams. The three beams traveling to the optical disk 10 via the half mirror 3, a collimator lens 4 and an object lens 5 are converged separately to form three convergence spots 100, 101 and 102 on the signal recording surface of the optical disk 10 as shown on the left side of FIG. 3. At this point, the positions of the three convergence spots 100, 101 and 102 have been adjusted properly so that irradiation positioned interval δ measured in the radial direction of the optical disk 10, a direction perpendicular to guide grooves 11 which are periodically formed on the recording surface of the optical disk 10, will be approximately ½ of the groove pitch Tp between the guide grooves 11 (hereafter, the guide groove pitch will also be called “track pitch (Tp)”) by rotating the diffraction grating 2 around its optical axis, for example. The three beams forming the convergence spots 100, 101 and 102 are reflected by the optical disk 10, and the reflected beams pass through the object lens 5 and the collimator lens 4 again and reach the half mirror 3. Part of light quantity of the beams is transmitted by the half mirror 3, and the transmitted beams are incident on a photodetector 20 through a detection lens 6.
In the photodetector 20, three photoreceptor surfaces 20a, 20b and 20c, each of which is divided into two or four parts, are arranged as shown on the right side of FIG. 3. The disk-reflected beams (beams reflected by the optical disk) are separately incident on corresponding photoreceptor surfaces and form detection beam spots 200, 201 and 202, respectively. A tracking error signal by the push-pull method (hereafter, simply referred to as “push-pull signal”) is obtained for each detection beam spot 200, 201, 202 by letting each subtracter 50a, 50b, 50c execute subtraction of photoelectric signals supplied from the photoreceptor surfaces.
Assuming that the detection beam spots 200, 201 and 202 correspond to the main beam spot 100, the sub spot 101 and the sub beam spot 102 on the optical disk 10 respectively and express push-pull signals obtained from the detection beam spots 200, 201 and 202 as Sa, Sb and Sc respectively, the phases of the push-pull signals Sb and Sc will obviously be different from the phase of the push-pull signal Sa by approximately 180° due to the positional relationship among the convergence spots 100, 101 and 102 on the optical disk 10. In other words, the push-pull signals Sa, Sb and Sa, Sc are outputted as antiphase waveforms (push-pull signals Sb and Sc are in phase). Thus, by adding the signals Sb and Sc, and subtracting the sum Sb and Sc from the signal Sa, an amplified tracking error signal can be obtained (not cancellation but amplification by the subtraction).
Meanwhile, the aforementioned displacement of the object lens and the tilt of the optical disk causes a certain offset component in each push-pull signal; however, such offset components in the push-pull signals Sa, Sb and Sc develop obviously in the same polarity regardless of the positions of the convergence spots 100, 101 and 102 on the disk surface. Therefore, by the aforementioned subtracting operation, the offset components contained in the push-pull signals selectively and advantageously cancel out, and consequently, an excellent tracking error signal from which the offset components have been removed perfectly or satisfactorily can be obtained.
As shown on the right side of FIG. 3, the push-pull signals Sb and Sc are added together by an adder 51, amplified properly by an amplifier 52, and subtracted by an subtracter 53 from the push-pull signal Sa regarding the main beam spot 100, by which the offset component contained in the push-pull signal Sa is removed perfectly or significantly and a high-quality tracking error signal with an enhanced amplitude is outputted.
The above is the signal detection principle adopted by the conventional DPP method. Incidentally, the conventional DPP method is a well-known technique which has been disclosed in JP-A-7-272303.
As explained above, the conventional DPP method has the advantage of precisely detecting the tracking error signal by use of a relatively simple detecting optical system while removing the offset component of the tracking error signal caused by the displacement of the object lens and the tilt of the optical disk perfectly or significantly, and is especially effective as a tracking error signal detection method for optical pickups that are adapted to optical disks having the periodically formed guide grooves.
However, the conventional DPP method explained above also involves the following problem in practical use. In the conventional DPP method, the irradiation positioned interval δ of the three convergence spots on the optical disk has to be adjusted to ½ of the track pitch Tp as mentioned before, by which the detection of satisfactory tracking error signals becomes difficult in cases of optical disks having track pitches Tp widely different from 26.
The recordable/rewritable optical disks in rapidly increasing demand have various types such as DVD-RAM, DVD-R and DVD-RW, in which DVD-RAM has two types: DVD-RAM1 (track pitch Tp: about 1.48 μm, storage capacity: about 2.6 GB) and DVD-RAM2 (track pitch Tp: about 1.23 μm, storage capacity: about 4.7 GB). Meanwhile, the track pitch is 0.74 μm in DVD-R and DVD-RW, which is exactly ½ of that of DVD-RAM1.
Recently, a versatile optical pickup, capable of reading/writing from/to all the various types of optical disks, is strongly awaited to come into practical use. In the conventional DPP method, however, if the irradiation positioned interval δ of the three convergence spots is adjusted and optimized for the tracking error signal detection from DVD-RAM, the irradiation positioned interval δ then become almost equal to the track pitch of DVD-R and DVD-RW, by which the tracking error signal detection from DVD-R and DVD-RW by the conventional DPP method becomes difficult. In short, if a single optical pickup is employed for various types of optical disks having different track pitches, the detection of satisfactory tracking error signals by the conventional DPP method might become difficult or impossible for some types of disks.
In order to address the above problem, there has recently been proposed a new tracking error signal detection method capable of handling any track pitch and consistently detecting the tracking error signal satisfactorily regardless of the difference in the track pitch while taking advantage of the merits of the conventional DPP method (e.g. JP-A-9-81942).
The tracking error signal detection method disclosed in JP-A-9-81942 employs almost the same detecting optical system as that of the conventional DPP method, except for the diffraction grating 2 for diffracting and separating the laser beam emitted by the laser light source into three beams. FIG. 4 shows a diffraction grating 2 employed in the document, in which the phase of periodic groove structure in a first half area 27 is shifted from that in a second half area 28 by approximately 180°. While description on the specific signal detection principle for this method described in JP-A-9-81942 is omitted here, by putting the diffraction grating 2 having the special periodic groove pattern at the position of the diffraction grating 2 shown in FIG. 1, even if the three convergence spots 100, 101 and 102 on the optical disk 10 are arranged in a single guide groove 11 as shown on the left side of FIG. 5 differently from the arrangement in the conventional DPP method, the push-pull signal Sa obtained from the detection beam spot 200 corresponding to the main beam spot 100 and the push-pull signals Sb and Sc obtained from the detection beam spots 201 and 202 corresponding to the sub spots 101 and 102 respectively, are outputted as antiphase waveforms (Sb and Sc are in phase) as shown on the right side of FIG. 5. Therefore, a tracking error signal totally equivalent to that in the conventional DPP method can be obtained using the same arithmetic circuitry.
The method explained above, the tracking error signal can be obtained by the differential push-pull method, similarly to the conventional DPP method, even if the three convergence spots are arranged in a single guide groove, the method enables consistent detection of an excellent tracking error signal from which the tracking offsets have been removed satisfactorily regardless of the difference in the disk track pitch.
As explained above, the new tracking error signal detection method disclosed in JP-A-9-81942 (hereafter, referred to as “in-line DPP method” for simplifying the explanation and clarifying the object of the present invention) has significant advantages in that it can resolve the problem with the conventional DPP method dependent on track pitch and realize consistent detection of excellent tracking error signals from various types of optical disks of different track pitches by a single optical pickup, while taking advantage of the merits of the conventional DPP method and employing similar arithmetic circuitry.
However, the in-line DPP method still involves a major problem in practical use as described below.
In the in-line DPP method, if the object lens is displaced or shifted in a direction substantially perpendicular to the recording tracks of the optical disk (hereafter, simply referred to as “tracking direction”), the amplitude of obtained tracking error signals drops significantly as the displacement increases. The deterioration of properties of the tracking error signal accompanying the object lens displacement in the tracking direction (hereafter called “object lens displacement-to-tracking error signal ratio characteristic”) is by far more remarkable in the in-line DPP method than in the conventional DPP method. Thus, in order to obtain a usable tracking error signal by the conventional in-line DPP method, the object lens displacement has to be limited to an extremely narrow permissible range, by which practical performance of the optical pickup is necessitated to be impaired seriously at present.
The above problem regarding the “object lens displacement-to-tracking error signal ratio characteristic” has not be mentioned in publicly known documents at all and, as a matter of course, no countermeasure has been disclosed.